Hydrothermal-assisted shearing exfoliation for few-layered MoS2 nanosheets

The exfoliation of bulk MoS₂ into few layers has attracted considerable attention as 2D nanomaterials in the past decade. We developed a facile approach for producing MoS₂ nanosheets by hydrothermal-assisted shearing exfoliation based on organic-free strategy. This original exfoliation was highly efficient for large-scale production and sustainable for the environment. The thickness of the as-exfoliated MoS₂ nanosheets was about 4–6 layers, and the lateral size became smaller from hydrothermal processing to shearing. The hydrothermal processing with the participation of ammonium carbonate played an important role in hydrothermal-assisted shearing exfoliation. As a prospective application, the antifriction performance of the as-exfoliated MoS₂ nanosheets in oil was evaluated using a ball-on-ball mode. Evidently, the average friction coefficient and wear scar diameter of 0.08 wt% MoS₂-based oil dropped to about 20.66% and 47.27% relative to those of the base oil, which exhibited an excellent antifriction and antiwear ability.

A facile exfoliation method based on hydrothermal-shearing exfoliation to obtain MoS₂ nanosheets.     

Mechanical properties of epoxy nanocomposites filled with melamine functionalized molybdenum disulfide

In this work, a melamine functionalized molybdenum disulfide (M-MoS₂) was prepared and used as fillers to form epoxy (EP)/MoS₂ nanocomposites. The effects of molybdenum disulfide (MoS₂) and melamine functionalized molybdenum disulfide (M-MoS₂) loading on the mechanical properties of epoxy composites were investigated and compared. With only addition of 0.8 wt% M-MoS₂, the tensile strength and modulus of EP/M-MoS₂ nanocomposites showed 4.5 and 4.0 times increase over the neat epoxy. Interestingly, the elongation at break value of EP was also increased with the introduction of M-MoS₂ fillers. These properties could result from the good dispersion and strong interfacial adhesion of M-MoS₂ fillers and the EP matrix. Therefore, this work provides a facile way to produce of high-performance EP nanocomposites.

In this work, a melamine functionalized molybdenum disulfide (M-MoS₂) was prepared and used as fillers to form epoxy (EP)/MoS₂ nanocomposites. The mechanical properties of EP were significantly improved, even with very low M-MoS₂ addition.     

Quantification of defects engineered in single layer MoS2

Atomic defects are controllably introduced in suspended single layer molybdenum disulfide (1L MoS₂) using helium ion beam. Vacancies exhibit one missing atom of molybdenum and a few atoms of sulfur. Quantification was done using a Scanning Transmission Electron Microscope (STEM) with an annular detector. Experimentally accessible inter-defect distance was employed to measure the degree of crystallinity in 1L MoS₂. A correlation between the appearance of an acoustic phonon mode in the Raman spectra and the inter-defect distance was established, which introduces a new methodology for quantifying defects in two-dimensional materials such as MoS₂.

We report on controllably creating and quantifying atomic defects with varying sulfur vacancies using helium ion irradiation in MoS₂.     

An integrated electrode based on nanoflakes of MoS2 on carbon cloth for enhanced lithium storage

Due to its high specific capacity (in theory), molybdenum disulfide (MoS₂) has been recognized as a plausible substitute in lithium-ion batteries (LIBs). However, it suffers from an inferior electric conductivity and a substantial volume change during Li⁺ insertion/extraction. By using a facile hydrothermal method, a flexible free-standing MoS₂ electrode has here been fabricated onto a carbon cloth substrate. The grafting of ultrathin MoS₂ nanoflakes onto the carbon cloth framework (forming CC@MoS₂), was shown to facilitate an improved electron transport, as well as an enhanced Li⁺ transport. As expected, the as-obtained CC@MoS₂ electrode was observed to exhibit an excellent lithium storage performance. It delivers a high discharge specific capacity of 2.42 mA h cm⁻² at 0.7 mA cm⁻² (even after 100 cycles), which is an impressive result.

Integrated carbon cloth@MoS₂ electrode is fabricated via a facile hydrothermal method as a binder-free anode for lithium-ion batteries.     

One-pot synthesized Cu/Au/Pt trimetallic nanoparticles as a novel enzyme mimic for biosensing applications

Multimetallic nanomaterials have aroused special attention owing to the unique characteristics of chemical, optical and enhanced enzyme mimetic capabilities resulting from the synergistic effect of different metal elements. In this work, we present a facile, gentle, fast and one-pot method for preparing Cu/Au/Pt trimetallic nanoparticles (TNPs), which possess intrinsic and enhanced peroxidase-like activity as well as excellent stability, sustainable catalytic activity, and robustness to harsh environments. Kinetic analysis indicated that Cu/Au/Pt TNPs exhibited strong affinities with H₂O₂ and 3,3,5,5-tetramethylbenzidine (TMB) as the substrates. To investigate the feasibility of Cu/Au/Pt TNPs-based strategy in biological analysis, H₂O₂ was chosen as a model analyte and a sensitive and specific detection for H₂O₂ was acquired with a detection limit of 17 nM. By coupling with glucose oxidase (GOD), this assay could also achieve a sensitive and selective detection of glucose with a detection limit of 33 μM, indicating the versatility of the method. In view of the potential combination with diverse enzyme-related reactions, the Cu/Au/Pt TNPs-based strategy is promising as a universal platform for biosensors.

A class of novel Cu/Au/Pt TNPs with enhanced peroxidase-like activity was developed and used as enzyme mimics for biosensing.     

Facile synthesis of vacancy-induced 2H-MoS2 nanosheets and defect investigation for supercapacitor application

In this paper, a 2D molybdenum disulfide (MoS₂) nanosheet is prepared via a one-step hydrothermal method as electrode material for supercapacitors. Meanwhile, a series of MoS_2−x nanostructures with sulfur vacancies have been successfully obtained in an Ar/H₂ mixed atmosphere at different annealing temperatures. The prepared materials were characterized by XRD, HR-TEM, Raman and XPS to identify their morphology and crystal properties. MoS_2−x assembled by interconnected nanosheets (MoS_2−x-700) provides a maximum specific capacitance of 143.12 F g⁻¹ at a current density of 1.0 A g⁻¹ with 87.1% of initial capacitance reserved after 5000 cycles. The outstanding performance of the annealed MoS_2−x nanosheets in sodium storage is mainly attributed to the synergistic effect of the unique interconnected structure and the abundant active vacancy generated by the sulfur vacancies. Atomic models of sulfur vacancy defects on the basal plane, Mo-edge and S-edge were established and the electronic properties of MoS_2−x were further evaluated assisted by first principles theory. DFT calculation results show that sulfur vacancy defects can provide additional empty states near the Fermi level and induce unpaired electrons, thus increasing the carrier density and improving electrical conductivity. Our findings in this work provide experimental and theoretical evidence of improving the electrochemical performance of 2H-MoS₂ nanosheets by annealing treatment.

In this paper, a 2D molybdenum disulfide (MoS₂) nanosheet is prepared via a one-step hydrothermal method as electrode material for supercapacitors.     

Chemical vapor deposition merges MoS2 grains into high-quality and centimeter-scale films on Si/SiO2

Two-dimensional molybdenum disulfide (MoS₂) has attracted increasing attention due to its promise for next-generation electronics. To realize MoS₂-based electronics, however, a synthesis method is required that produces a uniform single-layer material and that is compatible with existing semiconductor fabrication techniques. Here, we demonstrate that uniform films of single-layer MoS₂ can be directly produced on Si/SiO₂ at wafer-scale without the use of catalysts or promoters. Control of the precursor transport through oxygen dosing yielded complete coverage and increased connectivity between crystalline MoS₂ domains. Spectroscopic characterization and carrier transport measurements furthermore revealed a reduced density of defects compared to conventional chemical vapor deposition growth that increased the quantum yield over ten-fold. To demonstrate the impact of enhanced scale and optoelectronic performance, centimeter-scale arrays of MoS₂ photosensors were produced that demonstrate unprecedentedly high and uniform responsivity. Our approach improves the prospect of MoS₂ for future applications.

Control of the precursor transport through oxygen dosing yields increased MoS₂ coverage and increased connectivity between crystalline MoS₂ domains.     

Dual enzyme-like activity of iridium nanoparticles and their applications for the detection of glucose and glutathione

Here we show that iridium nanoparticles (Ir NPs) functionally mimic peroxidase and catalase. The possible mechanism of intrinsic dual-enzyme mimetic activity of Ir NPs was investigated. Based on the excellent peroxidase-like activity of Ir NPs, a new colorimetric detection method for reduced glutathione (GSH) and glucose was proposed.

Iridium nanoparticles could functionally mimic peroxidase and catalase. The possible mechanism of intrinsic dual-enzyme mimetic activity of Ir NPs was investigated.

Recently, nanomaterials with inherent enzyme-like activity have attracted considerable attention due to their simple preparation, storage, and separation, as well as the low cost as compared with natural enzymes. Various nanomaterials have been shown to mimic the activity of oxidase, peroxidase, catalase or superoxide dismutase (SOD), ranging from metals,^1–10 metal oxides,^11–15 and metal coordination complexes^16–21 to carbon-based nanomaterials.^22–26 The ability of these nanomaterials to replace specific enzymes may offer new opportunities for enzyme-based applications. For example, nanozymes with oxidase-like or peroxidase-like activity have shown potential applications in biosensing and immunoassay, such as the detection of H₂O₂, glucose, antioxidants, antigens, antibodies and so on.²⁷ By using nanozymes with peroxidase-like activities, Wei and co-workers recently have developed novel sensor arrays to detect biothiols and proteins as well as discriminate cancer cells owing to the differential nonspecific interactions between the components of the sensor arrays and the analytes, providing a potential approach to discriminate versatile analytes.⁸ Nanomaterials with SOD-like or catalase-like activity have exhibited antioxidant activity, thus could protect aerobic cells from oxidative stress, showing potential application in inflammation therapy.^6,28–31 Also, nanozymes with high catalase-like activity was able to produce O₂ at the hypoxic tumor site, serving as efficient agents for cancer therapy.^23,32,33 Very recently, Zhang'' group has developed a simple and biocompatible platform to elevate O₂ for improving photodynamic therapeutic efficacy by combining the photosensitizer with Prussian blue nanomaterials.³⁴ Prussian blue could catalyze H₂O₂ to generate O₂, and then the photosensitizer transforms the O₂ to produce singlet oxygen (¹O₂) upon laser irradiation for cancer therapy. Besides, Qu et al. have found the porous platinum nanoparticles with catalase-like activity, which greatly enhanced radiotherapy efficacy and overcame the hypoxic tumor microenvironment.³⁵In this work, we demonstrate that Ir NPs exhibited both peroxidase-like and catalase-like activities. As shown in Scheme 1, Ir NPs can catalyze the decomposition reaction of H₂O₂ into oxygen and water, possessing potential applications in the cancer therapy as catalase mimics. On the other hand, the tiny Ir NPs with the average diameter of 2.4 nm exhibited high peroxidase-like catalytic activity. Furthermore, by using H₂O₂ as an intermediary, a simple and sensitive colorimetric detection method for GSH and glucose has been designed.Open in a separate windowScheme 1Schematic presentation for dual-enzyme mimetic activity of Ir NPs.Synthesis of Ir NPs were carried out by a simple chemical reduction process, in which sodium hexachloroiridate(iii) hydrate was used as precursor with ascorbic acid as a protecting agent and sodium borohydride as the reducing agent (see Experimental section in ESI^†). After heating and stirring at 95 °C for 15 min, the resulting homogeneous light brown Ir NPs dispersion was obtained with good stability and reproducibility. The obtained Ir NPs was thoroughly characterized by various methods. Transmission electron microscopy (TEM) images indicated that the as-prepared Ir NPs showed a narrow size distribution with the average diameter of ∼2.4 nm (Fig. 1A–C). UV-vis spectrum of Ir NPs showed an absorption peak at ∼280 nm (Fig. 1D), in agreement with the value reported earlier,²⁸ indicating the formation of Ir NPs. XPS spectra of Ir 4f in Ir NPs were presented in the Fig. S1.^† A pair of doublet peaks at 61.0 and 64.0 eV were observed, revealing that Ir in Ir NPs is mostly metallic Ir(0).³⁶ Furthermore, inductively coupled plasma-optical emission spectroscopy (ICP-OES) disclosed that the exact concentration of Ir NPs is 25 μg mL⁻¹.Open in a separate windowFig. 1(A and B) TEM images of Ir NPs at different magnification. (C) Size distribution histogram of Ir NPs. (D) UV-vis absorption spectrum of Ir NPs. The inset shows the photograph of Ir NPs dispersed in the aqueous solution.To investigate the peroxidase-like activity of Ir NPs, the peroxidase coupled assay was employed and the change in absorbance of reaction was monitored using a UV-vis absorbance spectrophotometer. As shown in Fig. 2A, exposure of Ir NPs to a colorless peroxidase substrate 3,3′,5,5′-tetramethylbenzidine (TMB) in the presence of H₂O₂ resulted in the fast oxidation of TMB to a blue product. However, no color change of the TMB substrate was observed only with Ir NPs or H₂O₂, indicating the intrinsic peroxidase-like activity of Ir NPs. Similar to HRP, the catalytic activity of Ir NPs is dependent on the pH, temperature and catalyst dosage. Fig. S2^† showed that the catalytic activity of Ir NPs was much higher in weakly acidic solution and reached its highest at pH 4.0, consistent with those reported for peroxidase-like NPs and HRP.^37–40 In the range of 20–80 °C, the maximum catalytic activity was obtained under 50 °C. For simplicity, we adopted pH 4.0 and room temperature (∼20 °C) for subsequent analysis of peroxidase-like activity of Ir NPs. When increasing the dosage of Ir NPs, the catalytic activities of Ir NPs clearly increased as shown in Fig. 2B. Interestingly, some small gas bubbles were observed in the tubes at the same time.Open in a separate windowFig. 2(A) The absorption spectra and digital photos of different colorimetric reaction systems: (a) TMB + H₂O₂, (b) TMB + Ir NPs, and (c) TMB + H₂O₂+Ir NPs. (B) Time-dependent absorbance changes at 652 nm of TMB reaction solutions catalyzed by the different concentrations of Ir NPs. (C) Time-dependent absorbance changes at 240 nm of 20 mM H₂O₂ catalyzed by the different concentrations of Ir NPs incubated in 0.1 M HAc–NaAc buffer (pH 9.0). (D) The effect of concentration of Ir NPs on the formation of hydroxyl radical with terephthalic acid as a fluorescence probe. Reaction condition: 0.1 M HAc–NaAc buffer (pH 6.0).In order to confirm which gas was produced and whether Ir NPs had the intrinsic catalase-like activity, the decomposition of H₂O₂ was further investigated by monitoring the changes of UV-vis absorbance at 240 nm under the basic conditions. As shown in Fig. 2C, the absorbance was obviously decreased as the dosage of Ir NPs increased, and many gas bubbles could be observed in the cuvette, indicating Ir NPs could catalyze the decomposition of H₂O₂ into O₂. Temperature and pH can also make a big effect on the catalase-like activity of Ir NPs. Under the basic conditions or higher temperature, much more and bigger gas bubbles were produced (Fig. S3^†), suggesting higher catalase-like activity of Ir NPs. Obviously, Ir NPs possessed intrinsic catalase-like activity, which could be regulated by adjusting the temperature and pH.The formation of ˙OH during the reactions was assessed to better understand the mechanism for the dual enzyme-like activity of Ir NPs. Terephthalic acid was adopted here as a fluorescence probe to trap ˙OH. As shown in Fig. 2D, in the absence of Ir NPs, terephthalic acid emitted blue. However, the fluorescence intensity was gradually decreased as the concentration of Ir NPs increased, suggesting that Ir NPs could consume ˙OH radicals rather than generate ones. The reactivity of Ir NPs appears to be different from that of the other peroxidase mimics, where ˙OH mediates the oxidation of organic substrate.As Ir NPs exhibited the peroxidase-like activity, we monitored the reaction of Ir NPs with H₂O₂ and TMB. The apparent steady-state kinetic parameters were determined by changing one substrate concentration while keeping the other substrate concentration constant. The value ε = 39 000 M⁻¹ cm⁻¹ (at 652 nm) for the oxidized product of TMB was used here to obtain the corresponding concentration term from the absorbance data. As shown in Fig. S4,^† we observed that the oxidation reaction catalyzed by Ir NPs followed the typical Michaelis–Menten behavior toward both substrates. The Michaelis–Menten constant (K_m) and the maximum initial velocity (V_m) given in Table S1^† were obtained by using Lineweaver–Burk plot (Fig. S4B and D^†). Compared with the Pd–Ir cubes,³⁸ Ir NPs presented a similar K_m for TMB and a very low K_m for H₂O₂, suggesting that Ir NPs have a higher affinity to H₂O₂. Moreover, Ir NPs presented larger V_m for both of TMB and H₂O₂, indicating the strong catalytic activity of Ir NPs. What is more important is that Ir NPs showed high stability after long-term storage. After five months of storage at room temperature, the peroxidase-like catalytic activity of Ir NPs maintained 96% (Fig. S5^†), significantly expanding their practical applications.On the basis of the high affinity and catalytic activity of Ir NPs to H₂O₂, the analytes that could consume or produce H₂O₂ could be detected indirectly by using the TMB as substrate.²⁷ Therefore, a simple colorimetric method was developed to detect GSH and glucose using Ir NPs. GSH, which plays an important role in many cellular processes including redox activities, signal transduction, detoxification, and gene regulation,^41,42 can consume H₂O₂ and result in the shallowing color of the Ir NPs-TMB-H₂O₂ system. As presented in Fig. 3A, the absorbance dropped sharply with the addition of GSH. And a linear relationship between the absorbance and logarithmic values of GSH''s concentrations was obtained in the range from 200 nM to 100 μM (Fig. 3B). The level changes of GSH have been linked to varieties of diseases, such as diabetes, psoriasis, liver damage and Parkinsons.^43,44 The proposed colorimetric biosensor provided a sensitive method to monitor GSH. Furthermore, because H₂O₂ is the main product of the glucose oxidase (GOx)-catalyzed reaction; glucose could be detected based on the combination of GOx. Fig. 3C shows typical glucose concentration–response curves with the linear range of 10 μM to 2 mM. According to the principle of S/N = 3, the calculated detection limit was 5.8 μM. Table S2^† was listed to compare the sensing performance of Ir NPs with other nanomaterials. Ir NPs are superior to other nanomaterials in lower detection limit and wider detection range for colorimetric determination of glucose.Open in a separate windowFig. 3(A) Dose–response curve for GSH detection at 652 nm. (B) Linear relationship between the absorbance and logarithmic values of GSH''s concentrations in the range from 200 nM to 100 μM. (C) Dose–response curve for glucose detection at 652 nm. (D) Determination of the selectivity of glucose detection (from left to right: blank, 0.3 mM glucose, 3 mM fructose, 3 mM maltose, 3 mM lactose and 3 mM sucrose). Inset: the color change with the different solutions. The error bars represents the standard deviation of four measurements.To explore the selectivity of above glucose sensor, 10 times concentration of control samples including fructose, maltose, lactose and sucrose were tested as shown in Fig. 3D. The color difference could be distinguished by the naked eye, suggesting the high selectivity of the biosensing system for glucose detection. Using this method, we detected glucose in 50-fold dilution fetal bovine serum to demonstrate the feasibility of this biosensor for practical applications, and the results are listed in Table S3.^† As can be seen, the recoveries of glucose fall in the range of 93.3–104% by using the standard addition method. The proposed biosensor was also applied for determining glucose concentrations in blood samples donated by healthy and diabetic persons (Fig. S6^†). According to the calibration curve, the concentration of glucose from different samples was 7.0 mM and 14.4 mM, which agrees well with that measured in the local hospital, 6.8 mM and 14.4 mM. Therefore, this colorimetric method is suitable and satisfactory for glucose analysis of real samples with high sensitivity and selectivity.In summary, Ir NPs synthesized by a simple chemical reduction process exhibited both of peroxidase-like and catalase-like activity. Moreover, the dual enzyme-like activity could be regulated by adjusting the temperature and pH. On the one hand, Ir NPs could consume ˙OH radicals exhibiting potential applications in the antioxidant therapeutics as antioxidant nanozymes. On the other hand, as peroxidase mimics, Ir NPs were successfully applied in the construction of colorimetric biosensors to detect GSH and glucose. This work will facilitate the utilization of intrinsic dual-enzyme activity and other catalytic properties of Ir NPs in analytical chemistry, biotechnology, and medicine.     

Mesoporous MnFe2O4 magnetic nanoparticles as a peroxidase mimic for the colorimetric detection of urine glucose

Mesoporous MnFe₂O₄ magnetic nanoparticles (mMnFe₂O₄ MNPs) were prepared with a one-step synthesis method and characterized to possess intrinsic peroxidase-like activity, and had obvious advantages over other peroxidase nanozymes in terms of high catalytic affinity, high stability, mono-dispersion, easy preparation, and quick separation. The mMnFe₂O₄ MNPs were used as a colorimetric sensor for indirect sensing of urine glucose based on the sensing principle that H₂O₂ can be produced from glucose oxidation catalyzed by glucose oxidase (GOx), and under the catalysis of the mMnFe₂O₄ MNPs nanozyme, H₂O₂ can oxidize 3,3′,5,5′-tetramethylbenzidine (TMB) to produce a blue color in a few minutes. This sensor is simple, cheap, sensitive, and specific to glucose detection with a detection limit of 0.7 μM, suggesting its potential for on-site glucose detection.

Schematic illustration of glucose detection with glucose oxidase (GOx) and mMnFe₂O₄ MNPs-catalyzed system.     

A nano-sized Cu-MOF with high peroxidase-like activity and its potential application in colorimetric detection of H2O2 and glucose

Peroxidase widely exists in nature and can be applied for the diagnosis and detection of H₂O₂, glucose, ascorbic acid and other aspects. However, the natural peroxidase has low stability and its catalytic efficiency is easily affected by external conditions. In this work, a copper-based metal–organic framework (Cu-MOF) was prepared by hydrothermal method, and characterized by means of XRD, SEM, FT-IR and EDS. The synthesized Cu-MOF material showed high peroxidase-like activity and could be utilized to catalyze the oxidation of o-phenylenediamine (OPDA) and 3,3′,5,5′-tetramethylbenzidine (TMB) in the presence of H₂O₂. The steady-state kinetics experiments of the oxidation of OPDA and TMB catalyzed by Cu-MOF were performed, and the kinetic parameters were obtained by linear least-squares fitting to Lineweaver–Burk plot. The results indicated that the affinity of Cu-MOF towards TMB and OPDA was close to that of the natural horseradish peroxidase (HRP). The as-prepared Cu-MOF can be applied for colorimetric detection of H₂O₂ and glucose with wide linear ranges of 5 to 300 μM and 50 to 500 μM for H₂O₂ and glucose, respectively. Furthermore, the specificity of detection of glucose was compared with other sugar species interference such as sucrose, lactose and maltose. In addition, the detection of ascorbic acid and sodium thiosulfate was also performed upon the inhibition of TMB oxidation. Based on the high catalytic activity, affinity and wide linear range, the as-prepared Cu-MOF may be used for artificial enzyme mimics in the fields of catalysis, biosensors, medicines and food industry.

A Cu-MOF with high peroxidase-like activity was prepared and could be used for colorimetric detection of H₂O₂ and glucose with high selectivity and good linear range (50–500 μM).     

Molecular dynamics study of the frictional properties of multilayer MoS2

To reveal the friction mechanism of molybdenum disulfide (MoS₂), the frictional properties of multilayer MoS₂ lubrication film were studied under variable loads and shearing velocities by the molecular dynamics (MD) method. The results showed irreversible deformation of MoS₂ was caused by heavy load or high shear velocity during the friction process and the interlayer velocity changed from a linear to a ladder-like distribution; thus, the number of shear surfaces and the friction coefficient decreased. The low friction coefficient caused by heavy load or high velocity could be maintained with a decrease in load or velocity. For a solid MoS₂ lubrication film, the number of shearing surfaces should be reduced as much as possible and the friction pair should be run under heavy load or high shear velocity for a period of time in advance; thus, it could exhibit excellent frictional properties under other conditions. The proposed friction mechanism provided theoretical guidance for experiments to further improve the frictional properties of MoS₂.

Deformation of MoS₂ layers directly leads to decrease in potential and ultimately leads to decrease in friction coefficient.     

Nickel oxide decorated MoS2 nanosheet-based non-enzymatic sensor for the selective detection of glucose

Understanding blood glucose levels in our body can be a key part in identifying and diagnosing prediabetes. Herein, nickel oxide (NiO) decorated molybdenum disulfide (MoS₂) nanosheets have been synthesized via a hydrothermal process to develop a non-enzymatic sensor for the detection of glucose. The surface morphology of the NiO/MoS₂ nanocomposite was comprehensively investigated by field-emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HR-TEM), powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS) and Brunauer–Emmett–Teller (BET) analysis. The electro-catalytic activity of the as-prepared NiO/MoS₂ nanocomposite towards glucose oxidation was investigated by cyclic voltammetry, electrochemical impedance spectroscopy (EIS) and amperometry in 0.1 M NaOH. The NiO/MoS₂ nanocomposite-based sensor showed outstanding electrocatalytic activity for the direct electro-oxidation of glucose due to it having more catalytic active sites, good conductivity, excellent electron transport and high specific surface area. Meanwhile, the NiO/MoS₂ modified glassy carbon electrode (GCE) showed a linear range of glucose detection from 0.01 to 10 mM by amperometry at 0.55 V. The effect of other common interferent molecules on the electrode response was also tested using alanine, l-cysteine, fructose, hydrogen peroxide, lactose, uric acid, dopamine and ascorbic acid. These molecules did not interfere in the detection of glucose. Moreover, this NiO/MoS₂/GCE sensor offered rapid response (2 s) and a wide linear range with a detection limit of 1.62 μM for glucose. The reproducibility, repeatability and stability of the sensor were also evaluated. The real application of the sensor was tested in a blood serum sample in the absence and presence of spiked glucose and its recovery values (96.1 to 99.8%) indicated that this method can be successfully applied to detect glucose in real samples.

This study reported that NiO/MoS₂ based nanocomposite can be used as an electrocatalytic material to detect glucose with high selectivity in a blood serum.     

Preparation of MoS2-based polydopamine-modified core–shell nanocomposites with elevated adsorption performances

New molybdenum disulfide (MoS₂)-based core–shell nanocomposite materials were successfully prepared through the self-assembly of mussel-inspired chemistry. Characterization by Fourier transform infrared, thermogravimetric analysis, scanning electron microscope and transmission electron microscopy revealed that the surface of the flaked MoS₂ was homogeneously coated with a thin layer of polydopamine (PDA). Dye adsorption performances of the synthesized MoS₂–PDA nanocomposites were investigated at different pH values and reaction times. Compared with pure MoS₂ nanosheets, the obtained core–shell nanocomposites showed elevated adsorption performances and high stability, indicating their potential applications in wastewater treatment and composite materials.

New core–shell MoS₂–PDA nanocomposites are prepared via mussel-inspired chemistry and a simple interfacial self-assembly process, demonstrating potential applications in wastewater treatment and self-assembled core–shell composite materials.     

High sensitivity glucose detection at extremely low concentrations using a MoS2-based field-effect transistor

In recent years, molybdenum disulfide (MoS₂) based field-effect transistors (FETs) have attracted much attention because of the unique properties of MoS₂ nano-materials as an ideal channel material. Using a MoS₂ FET as a glucose solution biosensor has the advantages of high sensitivity and rapid response. This paper is concerned with the fabrication of a bilayer MoS₂-based FET and the study of its application in the high sensitivity detection of an extremely low concentration glucose solution. It was found that the source-drain current (I_ds) increases as the concentration of the glucose solution increases at the same gate voltage (V_gs) and drain voltage (V_ds). The sensitivity of the biosensor as high as 260.75 mA mM⁻¹ has been calculated and the detection limit of 300 nM was measured. The unknown concentration of a glucose solution was also detected using data based on the relationship between I_ds and glucose solution concentration. In addition, many significant advantages of the biosensor were observed, such as short response time (<1 s), good stability, wide linear detection range (300 nM to 30 mM) and the micro-detection of glucose solutions. These unique properties make the bilayer MoS₂-based FET a great potential candidate for next generation biosensors.

The high sensitivity (260.75 mA mM⁻¹) detection of an extremely low concentration (300 nM) glucose solution is demonstrated by the bilayer MoS₂ FET based biosensor.     

Construction of flower-like MoS2/Fe3O4/rGO composite with enhanced photo-Fenton like catalyst performance

High-performance and recyclable photocatalysts have attracted considerable amounts of attention for use in wastewater treatment. In this paper, a MoS₂/Fe₃O₄/rGO (0.1 wt%) composite was synthesized by an environmentally-friendly and facile strategy, and showed high potential for recyclability. The nanocomposite exhibited high photocatalytic activity in the presence of H₂O₂ and rGO (reduced graphene oxide) under visible-light irradiation. Notably, when 3 mg of MoS₂/Fe₃O₄/rGO (0.1 wt%) was added to rhodamine B (RhB, 30 mg L⁻¹) solution, the degradation rate was almost 100% within 40 min at neutral pH under visible-light irradiation. This rate was four times more rapid than that of MoS₂ and double that of MoS₂/Fe₃O₄. The results indicate that rGO plays an important role in photocatalysis by suppressing the recombination of photogenerated electron–hole pairs and enhancing the absorption capability of visible-light and organic dyes. Finally, the photocatalytic and stability mechanisms of MoS₂/Fe₃O₄/rGO (0.1 wt%) are proposed. This work further helps our understanding of the photo-Fenton mechanism. Furthermore, the synthesis of this composite has potential for application in energy storage devices.

Flower-like MoS₂/Fe₃O₄/rGO composites have been constructed, which exhibit highly efficient visible-light photocatalytic performance for removing of RhB in the presence of H₂O₂.     

Enhanced hydrogen evolution reaction activity of hydrogen-annealed vertical MoS2 nanosheets

Molybdenum disulfide (MoS₂) is a promising electrocatalyst for hydrogen evolution reaction (HER), but only edges and S-vacancies are catalytic active sites for the HER. Therefore, it is crucial to increase edge sites and S-vacancies for enhancing the HER activity of MoS₂. Here, we report an enhanced HER activity of MoS₂ by combing vertical nanosheets and H₂ annealing. Compared to horizontal MoS₂ nanosheets, pristine vertical MoS₂ nanosheets showed better HER activity due to a larger amount of edges. H₂ annealing further enhanced the HER activity of vertical MoS₂ nanosheets remarkably. Scanning electron microscopy (SEM), X-ray photoelectron spectra (XPS) and electrochemical impedance spectroscopy (EIS) were used to elucidate the enhanced HER activity by H₂ annealing. SEM images showed that H₂ annealing roughened the MoS₂ edges, leading to more edge sites. XPS data revealed the smaller S : Mo ratio after H₂ annealing, meaning more S-vacancies. Meanwhile, EIS measurements showed that charge transfer was accelerated by H₂ annealing. These findings elaborated the H₂ annealing induced enhancement of the HER activity, which were further confirmed by the subsequent re-sulfurization experiment.

Vertical configuration and H₂ annealing enhanced the hydrogen evolution reaction activity of MoS₂ nanosheets.     

Bandgap engineering of few-layered MoS2 with low concentrations of S vacancies

Band-gap engineering of molybdenum disulfide (MoS₂) by introducing vacancies is of particular interest owing to the potential optoelectronic applications. In this work, systematic density functional theory (DFT) calculations were carried out for few-layered 3R-MoS₂ with different concentrations of S vacancies. All results revealed that the defect energy levels introduced on both sides of the Fermi level formed an intermediate band in the band gap. Both the edges of the intrinsic and intermediate bands of the structures with the same type of vacancies were generally closer to the Fermi level, and the gaps decreased as the number of layers increased from 2 to 4. The preferentially formed S vacancies at the top layer and the transition of defect types from point to line led to similar indirect band gaps for 2- and 4-layered 3R-MoS₂ with a low bulk concentration (around 5%) of S vacancies. This is different from most reported results about transition metal dichalcogenide (TMD) materials that the indirect band gap decreases as the number of layers increases and the low concentrations of vacancies show negligible influence on the band gap value.

Band-gap engineering of molybdenum disulfide (MoS₂) by introducing vacancies is of particular interest owing to the potential optoelectronic applications.     

Highly efficient and flexible photodetector based on MoS2–ZnO heterostructures

Two-dimensional (2D) transition metal dichalcogenides (TMDs) such as molybdenum disulfide (MoS₂) and tungsten diselenide (WSe₂), have recently attracted attention for their applicability as building blocks for fabricating advanced functional materials. In this study, a high quality hybrid material based on 2D TMD nanosheets and ZnO nanopatches was demonstrated. An organic promoter layer was employed for the large-scale growth of the TMD sheet, and atomic layer deposition (ALD) was utilized for the growth of ZnO nanopatches. Photodetectors based on 2D TMD nanosheets and ZnO nanopatches were successfully fabricated and investigated, which showed a high photoresponsivity of 2.7 A/W. Our novel approach is a promising and effective method for the fabrication of photodetectors with a new structure for application in TMD-based transparent and flexible optoelectronic devices.

Two-dimensional transition metal dichalcogenides (TMDs) such as molybdenum disulfide, have recently attracted attention for their applicability as building blocks for fabricating advanced functional materials.     

Colorimetric detection of hydrogen peroxide and glucose by exploiting the peroxidase-like activity of papain

Papain, a natural plant protease that exists in the latex of Carica papaya, catalyzes the hydrolysis of peptide, ester and amide bonds. In this work, we found that papain displayed peroxidase-like activity and catalyzed the oxidation of 3,3′,5′,5′-tetramethylbenzidine (TMB) in the presence of H₂O₂. This results in the formation of a blue colored product with an absorption maximum at 652 nm. The effects of experimental parameters including pH and reaction temperature on catalytic activity of papain were investigated. The increase of absorbance induced by the catalytic effect of papain offers accurate detection of H₂O₂ in the range of 5.00–90.0 μM, along with a detection limit of 2.10 μM. A facile colorimetric method for glucose detection was also proposed by combining the glucose oxidase (GOx)-catalyzed glucose oxidation and papain-catalyzed TMB oxidation, which exhibited a linear response in the range of 0.05–0.50 mM with a detection limit of 0.025 mM. The method proposed here displayed excellent selectivity, indicating that common coexisting substances (urea, uric acid, ascorbic acid, maltose, lactose and fructose) in urine did not interfere with detection of glucose. More importantly, the suggested method was successfully used to precisely detect the glucose concentration in human urine samples with recoveries over 96.0%.

We reported a simple colorimetric method for the detection of glucose based on GOx-catalyzed glucose oxidation and papain-catalyzed TMB oxidation.     

Bimetallic gold and palladium nanoparticles supported on copper oxide nanorods for enhanced H2O2 catalytic reduction and sensing

The emergence of nanoscience and nanotechnology has revitalised research interest in using copper and its derived nanostructures to find exciting and novel applications. In this work, mono- and bimetallic gold and palladium nanoparticles supported on copper oxide nanorods (CuONRs) were prepared and their catalytic performance towards the reduction of H₂O₂ to form reactive oxygen radical species (ROS) was evaluated. The characterisation using microscopy and spectroscopic techniques confirms the successful synthesis of CuONRs, CuONRs@Au₆NPs, CuONRs@Pd₆NPs and CuONRs@Au₃Pd₃NPs. The efficient generation of ROS was confirmed using UV-vis spectroscopy and 1,3-diphenylisobenzofuran (DPBF) as a radical scavenger. The CuONRs possess excellent catalytic reduction activity for H₂O₂ by generating ROS. However, CuONRs also have lattice oxygens which do not participate in the catalytic reduction step. The lattice oxygens however allowed for the adsorption of gold and palladium nanoparticles (Au₆NPs, Pd₆NPs and Au₃Pd₃NPs) and thus enhanced catalytic reduction of H₂O₂ to produce ROS. The produced ROS was subsequently involved in the catalytic oxidation of a chromogenic substrate (TMB), resulting in blue coloured diimine (TMBDI) complex which was monitored using UV-vis and could also be observed using the naked eye. The catalyst dependence on pH, temperature, and H₂O₂ concentration towards efficient ROS generation was investigated. The gold and palladium-supported CuONRs nanocatalysts were evaluated for their potential applications in the fabrication of colorimetric biosensors to detect glucose oxidation by glucose oxidase (GOx). Glucose was used as a model analyte. The enzymatic reaction between GOx and β-d-glucose produces H₂O₂ as a by-product, which is then catalytically converted to ROS by the nanoparticles.

Mono- and bimetallic gold and palladium nanoparticles supported on copper oxide nanorods were prepared. Their catalytic performance towards the catalytic reduction of H₂O₂ to produce reactive oxygen radical species was evaluated.